The invention relates to an exhaust-gas turbine in an exhaust-gas turbocharger having a turbine wheel which is rotatably supported in a housing including an inlet channel for supplying exhaust gas to the turbine for driving the turbine wheel.
Document EP 0 654 587 B1 discloses an exhaust-gas turbocharger for an internal combustion engine, whose exhaust-gas turbine is arranged in the exhaust strand of the internal combustion engine and whose compressor is arranged in the intake tract. The turbine wheel of the exhaust-gas turbine is driven by the pressurized exhaust gases and its rotational movement is transmitted via a shaft to the compressor wheel of the compressor. Combustion air drawn in at ambient pressure is compressed by the compressor to an increased charge pressure. The exhaust-gas turbine is provided with a variable turbine geometry, composed of an axially movable guide vane structure arranged in the turbine inlet cross section between a turbine inlet channel and the turbine wheel. By means of the guide vane structure the turbine inlet flow cross section can be adjusted between a minimal, blocking, position and a maximum, open, position. The variable turbine geometry makes it possible to variably set the exhaust-gas counterpressure as a function of the present engine operating state.
The guide vane structure in the turbine inlet cross section is supported by an axial slide and forms one of the two lateral delimiting walls of the turbine inlet cross section. In the event of a movement of the axial slide, the free end side of the guide vane structure moves into opposite recesses in the turbine housing. A larger or smaller effective turbine inlet cross section is set depending on the axial position of the axial slide.
The adjustable delimiting wall which is formed by the axial slide in the turbine inlet cross section is situated on the side of the turbine remote from the outlet duct of the exhaust-gas turbine. When the turbine inlet cross section is fully open, the delimiting wall on the axial slide is situated in alignment with the axial end of the turbine wheel blades. In contrast, when the turbine inlet cross section is reduced, the delimiting wall on the axial slide is axially moved toward the opposite side of the turbine inlet and overlays part of the turbine wheel blades, as a result of which the flow conditions between the inlet channel and the turbine wheel change significantly in a way which disturbs the exhaust gas flow through the turbine.
WO 2004/048755 A1 discloses an exhaust-gas turbocharger having an exhaust-gas turbine to which exhaust gas can be supplied via an inlet channel, with a stationary guide vane structure with guide vanes being arranged in the transition from the inlet channel to the turbine wheel. In addition, an axial slide is arranged axially movably in the turbine inlet cross section, so that the effective turbine inlet cross section can be adjusted. The axial slide at the same time forms a contour sleeve which extends radially around the turbine wheel.
Integrated into the axial slide is a bypass which, when open, permits a discharge of exhaust gas out of the inlet channel directly into the outlet channel while bypassing the turbine wheel. The bypass is embodied as a passage in a wall section of the stationary turbine housing and is delimited by the outer lateral surface of the axial slide. When the axial slide is fully retracted, the passage communicates at one end with the inlet channel and at the other end with a discharge opening which leads to the outlet duct. When the axial slide is partially or fully pushed into the turbine inlet cross section, the discharge opening to the inlet duct is in contrast closed off by the axial slide.
When the axial slide is fully open, exhaust gas which flows via the bypass and the discharge opening directly into the outlet duct, is initially conducted radially into the bypass and must subsequently be deflected by 90° in the axial direction at the outer lateral surface of the axial slide, and thereafter perform a further 90° turn back in the radial direction in order to be able to exit through the discharge opening. With said multiple deflection of the exhaust gas, it is not possible to impart a swirl to the exhaust gas which would improve the gas flow through the turbine and therefore also the turbine efficiency.
WO 2004/113686 A1 also discloses an exhaust-gas turbine in an exhaust-gas turbocharger, having a turbine wheel which is radially enclosed by a contour sleeve into which an outflow opening is formed. Seated on the contour sleeve is an axial slide which is mounted movably on the contour sleeve so that when the axial slide is in a retracted position, the discharge opening is opened and exhaust gas can flow out of the inlet channel directly into the outlet duct of the exhaust-gas turbine while bypassing the turbine wheel.
A guide vane structure in the turbine inlet cross section is however not provided in said exhaust-gas turbine. Two inlet channels which are situated adjacent to one another, but are separated from one another are arranged in the turbine housing, which inlet channels are opened or closed off in succession by the axial slide.
There is however no vane structure with the result that the exhaust gas from the inlet channels impinges on the turbine wheel without an additional swirl.
It is the object of the present invention to provide an exhaust-gas turbine in an exhaust-gas turbocharger which has a variable turbine geometry in the form of a guide vane structure associated with an axial slide and which results in favorable flow conditions in a wide operating range and to provide a possibility for realizing an efficient exhaust gas discharge function of the exhaust-gas turbine with simple design measures.